There are many methods by which trace metals can be measured. Techniques that are useful for measuring trace metals in aqueous solutions include colorimetric methods, atomic absorption (AA) methods, inductively-coupled plasma (ICP) methods, X-ray fluorescence (XRF) methods; ion-selective electrode (ISE) methods, and stripping voltammetry (ASV, CSV, and PSA) methods.
Colorimetric measurements are useful when the concentration of the metal is relatively high (generally greater than 1 ppm) and are prone to interferences from common salts, sulfates, and other dissolved inorganic compounds. ISE methods are also not practical for ppb level measurements, particularly in solutions with many other metals present. XRF methods used in the field are useful at concentrations of 10 ppm and above. Although AA, ICP and laboratory-based XRF methods have each been used for the reliable measurement of trace metals below 1 ppb, only stripping voltammetry is suitable for unattended operation. The barriers to using AA, ICP, and XRF methods for these analyses are immense and include the expense to implement and the requirement for intensive support infrastructure.
Of the electrochemical methods, stripping voltammetry is the more sensitive. Stripping voltammetry takes place in either a two- or three-electrode electrochemical cell, which includes at least a working electrode and a reference electrode. The optional third electrode is called the “counter” or the “auxiliary” electrode. The auxiliary electrode is used when either the reference electrode has high internal electrical resistance or the solution has high resistivity. In either case, the auxiliary electrode is used in conjunction with a potentiostat to help compensate for parasitic voltage drops that appear across the electrical resistances in the measurement circuit.
Stripping voltammetry follows a conceptually simple procedure: (optionally) electrochemically clean the working electrode; setting the voltage on the working electrode (with respect to the reference electrode) to a deposition potential and accumulating the target analyte on the tip of the working electrode; ramping the working electrode voltage such that the deposited analyte is removed (stripped) by electrochemical reactions mediated by the working electrode and its potential.
While ramping the working electrode voltage, the electrical current that flows to the working electrode is measured and recorded. The presence of the target analyte, and in particular, the stripping of the analyte can be detected, monitored, and quantified through the measured current. The “ramp” function can be a linear increase (or decrease) with respect to time, it can be a staircase (digital) ramp, or it can take on a more complicated waveform such as in square wave stripping in which a periodic square wave voltage in added to the linear or digital ramp function.
Ultrasonic cleaning techniques are well known and ultrasonic techniques have been combined with stripping voltammetry for the purposed of in situ enhancement of the sensitivity and to keep the electrodes involved in the measurement clean. In-place sonication of electrodes have been described in U.S. Pat. Nos. 4,033,830 and 4,786,373. These patents describe the cleaning of electrodes for amperometric measurements and the direct ultrasonic excitation of the working electrode during the measurement. A considerable body of work concerning the use of ultrasound to enhance sensitivity of a trace metals measurement made via stripping voltammetry and to keep the electrode clean during a stripping voltammetry measurement in a sample that would otherwise foul the electrode sensing surface can be found in various scientific papers by Compton et al. A representative example summary of this work can be found in “Sonoelectroanalysis—an overview” by A. J. Saterlay and R. G. Compton, Fresenius J. Anal. Chem. 367:308-313 (2000).
A need exists for a practical, low-cost method for automatically monitoring trace metals in solution. Although several technologies exist for measuring trace metals, these methods are not very sensitive (they measure metals at non-trace concentrations) and are very expensive. None of the technologies offers the prospect of affordable long-term, unattended operation. The present invention seeks to fulfill this need and provides further related advantages.